Conventionally, various kinds of variable power optical systems that can be used in video cameras, broadcasting TV cameras, and similar devices, have been proposed. In particular, such devices have been proposed that include imaging lenses that include, in order from the object side, lens groups having positive, negative, positive, and positive refractive power, with the lens group having negative refractive power, the second lens group from the object side, moving toward the image side in order to perform zooming from the wide-angle end to the telephoto end. Such devices with such imaging lenses are shown, for example, in Japanese Laid-Open Patent Applications 2001-194590 and 2003-121737.
A varifocal lens is also known in a variable power optical system that does not use an interlocking mechanism in order to coordinate angle of view adjustment and focusing. Rather, the angle of view is adjusted by moving the variable power lens in the direction of the optical axis by rotating a varifocal ring installed on the lens barrel, and focusing error caused by the angle of view adjustment is corrected by moving a focusing lens in the optical axis direction by rotating a focus ring installed on the lens barrel, as shown, for example, in Japanese Laid-Open Patent Application H07-113942.
Recently, the demand for monitoring cameras for monitoring various kinds of facilities and roads has been increasing, especially the demand for the development of a variable power optical system that can be operated both day and night and that has a large range of variable focal lengths.
Because many conventional variable power optical systems vary the focal length by moving the second lens group from the object side along the optical axis toward the image side and correct the movement of the focal plane by moving the fourth lens group from the object side in an interlocked and coordinated manner with the second lens group, a mechanism to interlock the second lens group with the fourth lens group has become necessary. Such a mechanism is complex and prevents making the optical system as small as desired.
Simplifying the mechanism for moving the two lens groups by not using an interlocking mechanism between the second lens group and the fourth lens group in a variable power optical system may be considered, thus allowing the mechanism and the lens barrel generally to be made smaller. However, conventional variable power optical systems and varifocal lenses are not generally designed for use both during the day and at night, and, therefore, adequate correction of chromatic aberration in the range from the visible light region to the near-infrared light region is not made. Consequently, when the varifocal lens is focused in the visible light region, in order to use it in the near-infrared light region, focusing had to be performed again, complicating the operation. Also, in attempting to make the operation more versatile, expensive functions, such as autofocus functions, have had to be added.